Caption: Aurora borealis. Tree silhouetted against a green aurora borealis or northern lights display in the night sky. Aurorae, named after the Roman goddess of the dawn, are caused by the interaction of charged particles from the sun's solar wind with the Earth's magnetic field. The aurorae are seen at high latitudes, as these particles are channelled to the poles by the Earth's magnetic field. The particles collide with air molecules at heights from 80 to 1000 kilometres above the Earth. The colour of light emitted varies with air pressure and the gases involved, to give rise to flickering cascades of colour. Photographed in Finland.